A Novel Hsp90 Inhibitor Activates Compensatory Heat Shock Protein Responses and Autophagy and Alleviates Mutant A53T α-Synuclein Toxicity

A potential cause of neurodegenerative diseases, including Parkinson's disease (PD), is protein misfolding and aggregation that in turn leads to neurotoxicity. Targeting Hsp90 is an attractive strategy to halt neurodegenerative diseases, and benzoquinone ansamycin (BQA) Hsp90 inhibitors such as geldanamycin (GA) and 17-(allylamino)-17-demethoxygeldanamycin have been shown to be beneficial in mutant A53T alpha-synuclein PD models. However, current BQA inhibitors result in off-target toxicities via redox cycling and/or arylation of nucleophiles at the C19 position. We developed novel 19-substituted BQA (19BQA) as a means to prevent arylation. In this study, our data demonstrated that 19-phenyl-GA, a lead 19BQA in the GA series, was redox stable and exhibited little toxicity relative to its parent quinone GA in human dopaminergic SH-SY5Y cells as examined by oxygen consumption, trypan blue, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT), and apoptosis assays. Meanwhile, 19-phenyl-GAretained the ability to induce autophagy and potentially protective heat shock proteins (HSPs) such as Hsp70 and Hsp27. We found that transduction of A53T, but not wild type (WT) alpha-synuclein, induced toxicity in SH-SY5Y cells. 19-Phenyl- GA decreased oligomer formation and toxicity of A53T alpha-synuclein in transduced cells. Mechanistic studies indicated that mammalian target of rapamycin (mTOR)/p70 ribosomal S6 kinase signaling was activated by A53T but not WT alpha-synuclein, and 19-phenyl-GA decreased mTOR activation that may be associated with A53T alpha-synuclein toxicity. In summary, our results indicate that 19BQAs such as 19-phenyl-GAmay provide a means to modulate protein-handling systems including HSPs and autophagy, thereby reducing the aggregation and toxicity of proteins such as mutant A53T alpha-synuclein.